Getting jpeg quality? No clue!

Aron_Schatz

This one is really getting to me. I have functions for my site that'll create thumbnails and resize images and stuff, but I want the output quality to be the same if I add a watermark to it. Is there anyway I can find out the quality of a loaded image?

Thanks for any help you can give.

Weedpacket

Not really; the "quality" of a JPEG when it's saved is a measure of how much of the original data is kept - with the rest of course being discarded. When another application receives the file it can't tell whether information has been discarded or if the image really was like that originally. (Save a file with low quality from e.g., Photoshop; read the newly-saved image back in; save it again with a high quality setting - it doesn't improve, just becomes a bigger file 🙂).

There may be image-processing applications that store a comment block that contains quality information, but that is highly application-dependent.

Aron_Schatz

Thanks for the reply...

But, when I create a new image from gd, and then open it in Adobe, it'll show the quality (60). So, it is somewhere in the jpeg I assume?

Also, I made some regular images with photoshop and then opened them on another computer and it again knew the quality.

So it must know the quality from both files internally? I dunno...

ednark

here is something from a faqon jpegs

http://www.faqs.org/faqs/jpeg-faq/part1/

Most JPEG compressors let you pick a file size vs. image quality tradeoff by
selecting a quality setting. There seems to be widespread confusion about
the meaning of these settings. "Quality 95" does NOT mean "keep 95% of the
information", as some have claimed. The quality scale is purely arbitrary;
it's not a percentage of anything.

In fact, quality scales aren't even standardized across JPEG programs.
The quality settings discussed in this article apply to the free IJG JPEG
software (see part 2, item 15), and to many programs based on it. Some
other JPEG implementations use completely different quality scales.

Aron_Schatz

Okay, so there is basically no way...

So what should I do? If I add a watermark to an image of lower quality, it'll bump up the quality and create a file nearly twice the original size. On the other hand, if I wanted a high quality shot, I'd need to quality high after I merged the watermark.

This watermarking thing is not such a good thing after all 🙁.